1. Field of the Invention
The invention relates to a process of producing silicon bodies for solar cells and somewhat more particularly to an improved process for producing plate-or-tape-shaped silicon crystal bodies having crystalline pillar-like structures therein, which are equivalent to crystalline columnar structures, without melting of the base materials which form such bodies and which are useful for further processing into large-surface solar cells.
2. Prior Art
The above-referenced Grabmaier et al application (assigned to the instant assignee) discloses and claims a method of producing plate- or tape-shaped silicon crystal bodies having pillar-like structures therein, which are equivalent to crystal columnar structures, without melting of the base material forming such bodies, which are particularly suitable for further processing into large-surfaced solar cells.
Somewhat more particularly, this earlier described method comprises forming a slurry from an admixture of silicon powder comprised of silicon particles having an average grain size of less than about 1 .mu.m and a compatible liquid binder, extruding a relatively thin layer of such slurry with a suitable means, such as a tool for doctor-blading, on a base, drying such layer until it becomes essentially self-supporting and removing the base, and thereafter sintering such self-supporting slurry layer on a temperature-stable inert base in a protective gas atmosphere at a sintering temperature below about 1430.degree. C. in such a manner that a layer of monocrystalline silicon particles is generated which particles have an average particle diameter corresponding at least to the thickness of the dried slurry layer.
With the method described in the above-referenced Grabmaier et al application, the silicon slurry layer is sintered on a quartz glass base in an argon stream or atmosphere at a temperature of about 1350.degree. C. whereby silicon particles of less than 1 .mu.m diameter become consolidated and become so great that granules or particles having a diameter greater than that of the initial layer thickness (approximately 150 .mu.m) result.
However, crystal growth and particle consolidation is greatly obstructed by an oxide film (melting point greater than 1700.degree. C.) which envelops each silicon crystal or particle (melting point equal to about 1405.degree. C.) accordingly, additional oxygen supply at high temperatures, in particular, must be avoided.
High temperature treatment of silicon, such as melting or sintering, most often occurs in special furnaces which are coated with oxide materials, such as aluminum oxide and/or aluminum silicate. In instances where an argon atmosphere is utilized as a protective gas atmosphere during sintering or melting in such a furnace, an oxygen partial pressure at the elevated furnace temperatures, cannot be decreased below about 10.sup.-4 to 10.sup.-3 bar. This amount of oxygen is sufficient to generate an oxide film on the silicon crystals being treated and greatly obstructs crystal growth and, hence, the sintering process.